We present the ROSAT High Resolution Imager (HRI) data of the first resolved and detected classical nova shell in the X-ray wavelengths : The shell of Nova Persei (1901). We find that the shell is composed of knots and has an elliptical shape. The surface brightness is about 703.4+/-54.5 counts arcmin(-2). We estimate that the spectrum is of thermal origin with a luminosity of 8.0x10(31) erg s(-1) and an X-ray temperature of similar to 2.0x10(6) K in the 0.1-2.4 keV energy range. The knots are most likely a result of fragmentation and condensation in the post-shock region. This detection sheds some light into one of the most poorly understood stages of classical nova evolution. (C) 2000 COSPAR. Published by Elsevier Science Ltd.